Abstract
Abstract. Regional transport and heterogeneous reactions have been shown to play crucial roles in haze formation over a megacity cluster centered on Beijing. In this study, the updated Nested Air Quality Prediction Model System (NAQPMS) and the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model were employed to investigate the evolution of aerosols – in terms of the number concentration, size distribution, and degree of aging – in Beijing during six haze episodes between 15 November and 15 December 2016, as part of the Air Pollution and Human Health–Beijing (APHH-Beijing) winter campaign of 2016. The model exhibited reasonable performance not only with respect to mass concentrations of PM2.5 and its components in Beijing but also regarding the number concentration, size distribution, and degree of aging. We revealed that regional transport played a non-negligible role in haze episodes, with contributions of 14 %–31 % to the surface PM2.5 mass concentration. The contribution of regional transport to secondary inorganic aerosols was larger than that to primary aerosols (30 %–63 % and 3 %–12 %, respectively). The chemical transformation of SO2 along the transport pathway from source regions to Beijing was the major source of SO42-. We also found that sulfate formed outside Beijing from SO2 emitted in Beijing; this sulfate was then blown back to Beijing and considerably influenced haze formation. Along the transport pathway, aerosols underwent aging, which altered the mass ratio of the coating of black carbon to black carbon (RBC) and the size distribution of number concentrations. During the episodes, the geometric mean diameter (GMD) increased from less than 100 nm at the initial site to approximately 120 nm at the final site (Beijing), and the RBC increased from 2–4 to 4–8. During haze episodes with high humidity, the average contributions of gas and aqueous chemistry, heterogeneous chemistry, and primary emission to sulfate were comparable. However, their relative contributions varied with pollution levels. Primary emissions had the greatest impact under light to moderate pollution levels, whereas heterogeneous chemistry had a stronger effect under high pollution levels.
Highlights
In past decades, a megacity cluster in China that is centered on Beijing and includes 28 cities (272 500 km2, with a population of 191.7 million people) has experienced frequent severe and persistent haze episodes (Zhao et al, 2013; Sun et al, 2014, 2016)
The PM2.5 mass concentrations ranged from 75 to 115 μg m−3, indicating a light pollution level according to the Technical Regulation on Ambient Air Quality Index
In contrast to PIA, the secondary inorganic aerosol (SIA) contribution was dominated by regional transport of emissions from other cities (50 %–87 %)
Summary
A megacity cluster in China that is centered on Beijing and includes 28 cities (272 500 km, with a population of 191.7 million people) has experienced frequent severe and persistent haze episodes (Zhao et al, 2013; Sun et al, 2014, 2016). Using a ceilometer and in situ observation data, Zhu et al (2016) and Ma et al (2017) further proposed that regional transport was the major cause of the initial haze stage and that local chemistry, heterogeneous chemistry, dominated the subsequent later rise in Beijing. This result differs from the findings of modeling studies We believe that this study will help to understand the causes of haze in this megacity cluster
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
